Keep in mind that there's nothing stopping you from giving them an estimate! But you need to make it clear that most project scope changes come from client requests.
My co-founder spoke about this extensively at a conference several years back. It might be the best 15 minutes of your week:
http://www.youtube.com/watch?v=lIQIZ0NIkxk&hd=1
I am glad that fixed-price works for some, but I have a pretty hard line view on this: I think you're crazy. With a fixed price, either the client is paying too much or the developer isn't paid enough (and the work suffers due to decreased morale).
Every developer has a sad story about a client from hell and a fixed price engagement that crawled out of Scope Creep Alley. Yet, I'll bet that you've never heard anyone complaining about how they have to keep charging hourly to make pedantic changes.
As for the claim that hourly projects reward you for being a slower developer:
- there's nothing wrong with being a slow developer, assuming you're slow for good reasons like careful design and thoughtful testing
- this slur only really applies to time and materials developers that aren't busy! a good developer is likely busy, which means there's more work in the can and no reason to drag your heels in a dishonest way
Finally, the best ideas for a project come while you're working on it. How do you change course on a fixed-price budget? Are you going to stop to renegotiate a contract?
Sorry if I seem "passionate" about this. Building Unspace over the past six years into a happy place to work that only does T&M was a direct response to the ten previous years of fixed price hell. If this post saves one person from doing fixed price bids, my work here is done.